This invention relates to a ringer circuit for a telephone instrument. Electronic tone ringers for use in telephone instruments are well-known, dating from such U.S. Pat. Nos. as 3,772,470 issued Nov. 13, 1973 and 3,867,585 issued Feb. 18, 1975. In such patents, it is conventional to provide a rectifier bridge across the line to which ring signals are fed. It is also conventional to provide threshold circuits, voltage clamp circuits and transient suppression in the circuit between the bridge and the oscillator. In the previously noted references, the oscillator output was forwarded to a transducer such as a speaker. In other known circuits such as U.S. Pat. No. 4,214,131 issued July 22, 1980 and 4,140,884 issued Feb. 20, 1979, audio output is produced by a piezo-electric transducer.
In various prior art electronic ringer arrangements which are powered from ringing signals received over the telephone line, the incoming ringing signals are coupled to a full wave bridge rectifier which converts the a.c. ringing voltage to d.c. Various arrangements are utilized to ensure that the output voltage of the bridge rectifier is limited so that the remaining portion of the ringer is protected from high voltages. One approach is to connect a zener diode across the output terminals of the bridge rectifier. This approach, however, results in added component count.
Additionally, certain prior art ringer arrangements include a zener diode in series with the output of the bridge rectifier so that low level AC signals such as voice signals, DTMF signaling, etc. will not turn the ringer on. A disadvantage of these arrangements is that the serial connection of the zener diode reduces the power available to drive the ringer circuit when ringing signals are present.
Further, under the EIA STANDARDS PROPOSAL, PN-1286, Phase 1, Telephone Instruments for Voiceband Applications with Loop Signaling, Sixth Draft, Sept. 1979 prepared by the EIA Engineering Committee TR-41 a non-linear impedance "signature" is required of the ringer circuits for loop maintenance testing. The "signature" required is that the input impedance at 2.5 VRMS at 24 Hz must be more than four times the impedance at 10 VRMS. It is believed that none of the prior art electronic ringers provide such a signature.
A further problem with certain prior art electronic ringers is that during dialing of digits, so called "bell tapping" tone bursts are produced.